An air-treatment system for a confined environment, in particular the cabin of a vehicle, includes: a supply unit including an inlet port, a delivery port, and a purge port, the supply unit being configured for delivering air to said confined environment through the delivery port; and a first filtering device having a first filtration level, and a second filtering device having a second filtration level, each of the first and second filtering devices being arranged in fluid communication with the inlet port of said supply unit at a circuit node. A single flow-control valve determines inlet flow through one or other of the filtering devices.